In textile finishing, vat dyes for dyeing cellulose fibers have a significant share of the marked (approx. 12.5%, world consumption approx. 25,000 tons per year). In particular, owing to the high fastness, this class of dyes belongs to the high-grade dyes. When used in dyeing, the insoluble dye particles that have primarily no fiber affinity are converted by reduction into their alkali-soluble leuco form. The reduced dye has a high affinity for the substrate and at this stage attaches rapidly to the product to be dyed. When the absorption phase has ended, the leuco form is oxidized in order to fix the dye, thus forming the water insoluble pigment. The dyes are, in their basic chemical structure, frequently anthraquinonoid or indigoid types. Sulfur dyes are inferior to vat dyes from a qualitative point of view, but price-wise are very good, so that they have a relatively large share of the market in cellulose dyeing (25%, 50,000 tons per year).
Sulfur dyes are used analogously to vat dyes, where the reduction of sulfur dyes is possible at lower redox potentials.
Many textile dyes of other classes of dyes contain azo groups in the dyeing portion of their molecule. These azo groups can be split irreversibly through reduction, a feature that can be exploited to disintegrate the dyes (stripping and correction of faulty dyeings).
Reducing agents are also added to destroy excess bleaching agents, for reductive bleaching (wool) and reductive waste water treatment (decolorization).
The main reducing agent for vat dyeing and for reductive splitting of azo dyes is Na.sub.2 S.sub.2 O.sub.4 (sodium dithionite) ("hydro"), which exhibits a reduction potential of approx. -1000 mV in an alkaline medium. Derivatives of sulfinic acid (BASF Rongalit types) are added for reductions at higher temperatures (vapor processes, high temperature (HT) process) (reduction potential at 50.degree. C. approx. -1000 mV). Derivatives of sulfinic acid can be activated by the addition of heavy metal compounds such as Ni cyano-complexes, Co complexes, etc.. The addition of anthraquinone compounds as accelerator for the added reducing agents has been proposed, but is not carried out very much in practice.
Other reducing agents are thiourea dioxide (-1000 mV), hydroxyacetone (-810 mV) and sodium hydridoborate (-1,100 mV). Indigo lies, with respect to the requisite reduction potential (approx. -600 mV), between the vat dyes and the sulfur dyes. Here, in addition to "hydro", hydroxyacetone/sodium hydroxide solution can also be added as the reducing agent. Historically, ferrous sulfate (FeSO.sub.4)-lime-vats, zinc-lime-vats and fermentation vats were added.
For sulfur dyeing, other reducing agents can also be used on account of the lower requisite reduction potential. The main reducing agents are Na.sub.2 S and NaHS (reduction potential approx. -500 mV). Mixtures of glucose/sodium hydroxide solution have also been added.
In various Indian studies (cf. E. H. Daruwalla in Textile Asia, September 1975, pages 165 ff) a process of the kind characterized above has been proposed in which the consumption of sodium dithionite is reduced by the application of a direct voltage. This reduction can be traced to the fact that the reducing agent at the cathode is converted into a form that exhibits increased reducing power. Due to the reaction with the dye, this substance decomposes into the same products as the sodium dithionite itself. These products cannot be regenerated at the applied voltage at the cathode. Thus, this voltage is any way at a height that can be used only at the mercury electrode that is used, but for electrode materials that can be used in practice would lead to the generation of harmful hydrogen.
The reducing agents added at that time have various drawbacks when they are applied. Na.sub.2 S.sub.2 O.sub.4, is a relatively expensive chemical, which must be imported from many countries. In the dyeing procedures, a large excess of Na.sub.2 S.sub.2 O.sub.4, based on the volume required in theory for reduction, must be added. In the dye bath the oxygen present in the liquor must first be removed since not until then can the dye reduction begin. During the dyeing process, Na.sub.2 S.sub.2 O.sub.4 is consumed continuously by atmospheric oxygen from the environment. The volume required ranges from approx. 1.25 to 2.5 kg reducing agent per kg of dye.
The high volume required results in an accumulation of oxidation products of the reducing agent in the dye liquor. Reuse of the dye liquor is possible only in a minimum of cases. The quantity of reducing agent in the dye bath to the end of the dyeing process must suffice for total reduction. Therefore, the dye bath is drained with a relatively large quantity of reducing agent. Therefore, oxidation takes place in a new treatment bath, since otherwise the entire excess reducing agent that is still present must also be oxidized in the dye bath.
The reducing agent bath leads, in the waste water, to a significant consumption of oxygen, a state that leads to waste water problems. When sulfides are used as the reducing agent, the cost of procurement is relatively low; however, the waste water problem gains increasingly in importance, since here, in addition to the consumption of oxygen, significant toxicity and odor-related problems arise.